The present invention relates to the field of optical communications, and more particularly, to the delivery of broadcast services using broadband optical sources and pre-compensation of dispersion.
The distribution of broadcast services over local access passive optical networks (PONs) that carry switched services was proposed years ago, see R. C. Menendez, S. S. Wagner, and H. L. Lemberg, xe2x80x9cPassive Fiber Loop Architecture Providing Both Switched and Broadcast Transport,xe2x80x9d Electron. Lett., vol. 26, pp. 273-274 (1990). This topic was re-examined recently, see P. P. Iannone, K. C. Reichmann, and N. J. Frigo, xe2x80x9cBroadcast Digital Video Delivered over WDM Passive Optical Networks,xe2x80x9d IEEE Phot. Technol. Lett., vol. 8, pp. 930-932 (1996), largely due to the emergence of Waveguide Grating Routers (WGRs) with periodic transmission functions. See C. Dragone, xe2x80x9cAn Nxc3x97N Optical Multiplexer Using a Planar Arrangement of Two Star Couplers,xe2x80x9d IEEE Phot. Technol. Lett., vol. 3, pp. 812-815 (1991); and M. K. Smit, xe2x80x9cNew Focusing and Dispersive Planar Component Based on an Optical Phased Array,xe2x80x9d Elect. Lett., vol. 24, pp. 385-386 (1988). Specifically, the properties of the WGR permit it to deliver both digital broadcast and switched services on the same fiber-optic infrastructure. Using sources with different spectral widths changes the character of a WDM PON from a point-to-point link (for line sources) to a broadcast star (for broadband optical sources), N. J. Frigo, K. C. Reichmann, and P. P. Iannone, xe2x80x9cWDM Passive Optical Networks: A Robust and Flexible Infrastructure for Local Access,xe2x80x9d in Photonics Networks , G. Prati, ed., pp. 201-212, Springer-Verlag (1996). This attribute leads to a flexible way to provide different services and service types, simultaneously, over a single PON.
A shortcoming of this approach is that the communication band which is the most natural to use for such services is also the band that is the least tolerant of broad optical sources operating at high bandwidths. This band, the 1500 nm region, has the advantages of both low-loss transmission and ready availability of Erbium-Doped Fiber Amplifiers (EDFA) that make delivery of a common, broadcast, signal attractive. With these attractive features comes the disadvantage that there is a significant amount of dispersion when signals that are broadband both in information content (i.e. high modulation frequency content) and optical spectral content propagate on conventional optical fiber. This can limit the effective range of such systems to a few kilometers.
This dispersion penalty is fundamental for broadband sources and will apply to delivery of such signals regardless of whether delivered over a network with wavelength-division-multiplexing components or over a more conventional broadcast star. While dispersion compensating fiber (DCF) has been used in conventional long-haul optical transmission systems, it would be cost-prohibitive to use a DCF for each PON or feeder line of the distribution system. Moreover, dispersion is not generally a problem for PONs because a line source (or laser) is typically used rather than a broadband source.
Therefore, there is a need for a technique to provide broadcast services over a WDM PON while economically compensating for dispersion.
A broadcast delivery system is provided that includes a plurality of passive optical networks (PONs), each PON includes at least an optical transmitter and a feeder line connected thereto. The system also includes a broadband optical source outputting a broadband optical signal. A dispersion compensating fiber has a first end connected to the broadband optical source and a second end connected to each of the plurality of PONs (or feeder lines). The dispersion compensating fiber dispersion compensates for each of the feeder lines in the plurality of PONs within an acceptable range. Therefore, a plurality or group of feeder lines are pre-compensated using one DCF to share the DCF plant costs across a larger number of subscribers.